1. Field of the Invention
The present invention pertains to a foamable PVC resin formulation and a process of foaming. The product of the invention is useful as a gasket material for acrylic taillights.
2. Description of the Prior Art
Gasket materials formed from foamed PVC formualtions which are adapted to be used adjacent acrylic taillights in autombiles and trucks cannot contain any plasticizer which will exert a solvating effect on the acrylic material forming the taillight. It has been common for the automotive industry to therefore avoid the use of such common polyvinyl chloride plasticizers as the phthalates or phosphates which do exert such a solvating effect. Two classes of plasticizer which are useful in such foamed products are the polyester and epoxy plasticizers. It has been common for PVC homopolymers to be used in prior art foamable formulations in combination with such plasticizers and with a nitroso blowing agent (e.g., N,N-dimethyl-N,N' -dinitrosoterephthalamide which is available in a 70%, by weight, strength in white mineral oil under the trademark "Nitrosan" from E. I. Du Pont de Nemours & Co.) Such prior art formulations can be readily foamed to form a foam having a smooth, unblistered surface and a medium foam density. However if other blowing agents are used under similar foaming conditions, e.g., azodicarbonamide blowing agents, a foamed product havng bad surface blisters is produced.
The use of such a dialkyl dinitrosoterephthalamide can cause problems. In order to get a reasonably low density foam using such blowing agents a two step process, e.g., a preliminary gelling step followed by an expansion step, is often needed. Storing of such blowing agents can also be a problem since even at room temperature the active ingredient contained in such formulations will decompose with the evolution of gas. Additionally, in foamable formulations containing basic ingredients (e.g., methylamine, ethylenediamine, alcoholic solutions of caustic soda, potash or sodium hydroxide) the active ingredient can react and evolve diazomethane. The residue of such a decomposition reaction is dimethylterephthalate which has a limited compatibility with the polyvinyl chloride foam system thereby requiring a separation procedure. A need therefore has arisen in the art for alternative blowing agent and resin systems for such applications.